.TH std::partial_ordering 3 "2024.06.10" "http://cppreference.com" "C++ Standard Libary"
.SH NAME
std::partial_ordering \- std::partial_ordering

.SH Synopsis
   Defined in header <compare>
   class partial_ordering;      \fI(since C++20)\fP

   The class type std::partial_ordering is the result type of a three-way comparison
   that:

     * Admits all six relational operators (==, !=, <, <=, >, >=).
     * Does not imply substitutability: if a is equivalent to b, f(a) may not be
       equivalent to f(b), where f denotes a function that reads only
       comparison-salient state that is accessible via the argument's public const
       members. In other words, equivalent values may be distinguishable.
     * Admits incomparable values: a < b, a == b, and a > b may all be false.

.SH Constants

   The type std::partial_ordering has four valid values, implemented as const static
   data members of its type:

   Member constant              Definition
   less(inline constexpr)       a valid value of the type std::partial_ordering
   \fB[static]\fP                     indicating less-than (ordered before) relationship
                                \fI(public static member constant)\fP
                                a valid value of the type std::partial_ordering
   equivalent(inline constexpr) indicating equivalence (neither ordered before nor
   \fB[static]\fP                     ordered after)
                                \fI(public static member constant)\fP
   greater(inline constexpr)    a valid value of the type std::partial_ordering
   \fB[static]\fP                     indicating greater-than (ordered after) relationship
                                \fI(public static member constant)\fP
   unordered(inline constexpr)  a valid value of the type std::partial_ordering
   \fB[static]\fP                     indicating relationship with an incomparable value
                                \fI(public static member constant)\fP

.SH Conversions

   std::partial_ordering cannot be implicitly converted to other comparison category
   types, while both std::strong_ordering and std::weak_ordering are
   implicitly-convertible to partial_ordering.

.SH Comparisons

   Comparison operators are defined between values of this type and literal 0. This
   supports the expressions a <=> b == 0 or a <=> b < 0 that can be used to convert the
   result of a three-way comparison operator to a boolean relationship; see std::is_eq,
   std::is_lt, etc.

   These functions are not visible to ordinary unqualified or qualified lookup, and can
   only be found by argument-dependent lookup when std::partial_ordering is an
   associated class of the arguments.

   The behavior of a program that attempts to compare a partial_ordering with anything
   other than the integer literal 0 is undefined.

   operator==
   operator<
   operator>   compares with zero or a partial_ordering
   operator<=  \fI(function)\fP
   operator>=
   operator<=>

operator==

   friend constexpr bool operator==( partial_ordering v, /*unspecified*/ u )       \fB(1)\fP
   noexcept;
   friend constexpr bool                                                           \fB(2)\fP
       operator==( partial_ordering v, partial_ordering w ) noexcept = default;

.SH Parameters

   v, w - std::partial_ordering values to check
   u    - an unused parameter of any type that accepts literal zero argument

.SH Return value

   1) true if v is equivalent, false if v is less, greater, or unordered
   2) true if both parameters hold the same value, false otherwise

operator<

   friend constexpr bool operator<( partial_ordering v, /*unspecified*/ u )        \fB(1)\fP
   noexcept;
   friend constexpr bool operator<( /*unspecified*/ u, partial_ordering v )        \fB(2)\fP
   noexcept;

.SH Parameters

   v - a std::partial_ordering value to check
   u - an unused parameter of any type that accepts literal zero argument

.SH Return value

   1) true if v is less, and false if v is greater, equivalent, or unordered
   2) true if v is greater, and false if v is less, equivalent, or unordered

operator<=

   friend constexpr bool operator<=( partial_ordering v, /*unspecified*/ u )       \fB(1)\fP
   noexcept;
   friend constexpr bool operator<=( /*unspecified*/ u, partial_ordering v )       \fB(2)\fP
   noexcept;

.SH Parameters

   v - a std::partial_ordering value to check
   u - an unused parameter of any type that accepts literal zero argument

.SH Return value

   1) true if v is less or equivalent, and false if v is greater or unordered
   2) true if v is greater or equivalent, and false if v is less or unordered

operator>

   friend constexpr bool operator>( partial_ordering v, /*unspecified*/ u )        \fB(1)\fP
   noexcept;
   friend constexpr bool operator>( /*unspecified*/ u, partial_ordering v )        \fB(2)\fP
   noexcept;

.SH Parameters

   v - a std::partial_ordering value to check
   u - an unused parameter of any type that accepts literal zero argument

.SH Return value

   1) true if v is greater, and false if v is less, equivalent, or unordered
   2) true if v is less, and false if v is greater, equivalent, or unordered

operator>=

   friend constexpr bool operator>=( partial_ordering v, /*unspecified*/ u )       \fB(1)\fP
   noexcept;
   friend constexpr bool operator>=( /*unspecified*/ u, partial_ordering v )       \fB(2)\fP
   noexcept;

.SH Parameters

   v - a std::partial_ordering value to check
   u - an unused parameter of any type that accepts literal zero argument

.SH Return value

   1) true if v is greater or equivalent, and false if v is less or unordered
   2) true if v is less or equivalent, and false if v is greater or unordered

operator<=>

   friend constexpr partial_ordering operator<=>( partial_ordering v,              \fB(1)\fP
   /*unspecified*/ u ) noexcept;
   friend constexpr partial_ordering operator<=>( /*unspecified*/ u,               \fB(2)\fP
   partial_ordering v ) noexcept;

.SH Parameters

   v - a std::partial_ordering value to check
   u - an unused parameter of any type that accepts literal zero argument

.SH Return value

   1) v.
   2) greater if v is less, less if v is greater, otherwise v.

.SH Notes

   The built-in operator<=> between floating-point values uses this ordering: the
   positive zero and the negative zero compare equivalent, but can be distinguished,
   and NaN values compare unordered with any other value.

.SH Example

    This section is incomplete
    Reason: no example

.SH See also

   strong_ordering the result type of 3-way comparison that supports all 6 operators
   (C++20)         and is substitutable
                   \fI(class)\fP
   weak_ordering   the result type of 3-way comparison that supports all 6 operators
   (C++20)         and is not substitutable
                   \fI(class)\fP

.SH Category:
     * Todo no example
